This invention relates to downhole drilling, specifically downhole drilling for oil, gas, geothermal and horizontal drilling. More specifically, the invention relates to logging-while-drilling methods using a pulse neutron generator and detectors. Also, the invention relates to a method for a secondary nuclear measurement while drilling.
The prior art discloses several improvements for obtaining nuclear measurements downhole. U.S. Pat. No. 7,284,605, which is herein incorporated by reference for all that it contains, discloses a method for reducing stand-off effects of a downhole tool includes disposing the downhole tool in a borehole, wherein the downhole tool comprises at least one moveable section disposed between an energy source and a receiver on the downhole tool; and activating the at least one moveable section to reduce a thickness of at least one selected from a mud layer and a mudcake between the downhole tool and a wall of the borehole.
U.S. Pat. No. 6,666,285, which is herein incorporated by reference for all that it contains, discloses a logging-while-drilling gamma ray back scatter density system with elements configured to minimize material between sensor and the borehole environs, maximize shielding and collimation efficiency, and increase operational reliability and ruggedness. The system comprises a drill collar with a cavity in the outer wall, and an instrument package containing a sensor. The instrument package is disposed in the cavity and protrudes from the outer wall of the collar. Embodied as a density LWD system, the sensor consists of a gamma ray source and two detectors mounted within an instrument package framework made of high Z shielding material. A stabilized containing an alignment channel in the inner surface is disposed around the collar and receives the protrusion.
U.S. Pat. No. 5,250,806, which is herein incorporated by reference for all that it contains, discloses an apparatus and method for measuring density, porosity and other formation characteristics while drilling is disclosed. The apparatus, preferably housed in a drill collar and placed within a drill string, includes a source of neutrons and a source of gamma rays placed within a tubular body which is adapted to provide for the flow of drilling through it. Two sets of stabilizer blades are provided. One set, associated with the neutron source, includes secondary radiation detectors that are placed radially beyond the nominal outer radius of the body. Formation porosity measurement accuracy is substantially enhanced since the standoff of the detectors from the formation is substantially decreased. Another set, associated with the gamma ray source, includes one or more gamma ray detection assemblies in a single blade. Each of the gamma ray detector assemblies is also placed radially beyond the nominal outer radius of the tubular wall.
U.S. Pat. No. 5,242,020, which is herein incorporated by reference for all that it contains, discloses an extending arm is incorporated into a formation evaluation MWD collar or sub for extending outwardly from the tool and maintaining direct and continuous contact with the borehole wall (e.g., formation). In accordance with this invention, a method is presented for intermittently deploying the extendable arm and thereby decreasing drilling interference (caused by the arm) and avoiding the damage caused by accidents involving a nuclear source.
The prior art also discloses means for securing equipment in downhole tool string components, such as that disclosed in U.S. Pat. No. 7,299,867, which is herein incorporated by reference for all that it contains. This patent discloses a hanger mounted within a bore of a tubular string component has a split ring, a tapered key and a passageway formed in the hanger. The split ring has interfacial surfaces cooperating with interfacial surfaces of the tapered key.